docs: def ensure_alt_id_in_long_form(alt_id_col, long_form):
      """
      Ensures alt_id_col is in long_form, and raises a ValueError if not.
  
      Parameters
      ----------
      alt_id_col : str.
          Column name which denotes the column in `long_form` that contains the
          alternative ID for each row in `long_form`.
      long_form : pandas dataframe.
          Contains one row for each available alternative, for each observation.
  
      Returns
      -------
      None.
      """
      if alt_id_col not in long_form.columns:
          msg = "alt_id_col == {} is not a column in long_form."
          raise ValueError(msg.format(alt_id_col))
  
      return None